As a conventional image acquisition device, there is, for example, the device described in Patent Literature 1. In this device, light from a specimen is split into two beams by a half prism and the beams are received by a photoelectric conversion element consisting of a two-dimensional imaging element such as a CCD area sensor. A control circuit for the photoelectric conversion element has a scan area setting unit capable of setting two arbitrary scan areas for two-dimensional scanning on a light receiving surface. Then, focusing control is executed based on a focus deviation signal obtained from beams received in the two scan areas set by the scan area setting unit and an entire or partial image of the specimen is taken.